This invention relates generally to milling machines and specifically to means for rapidly and easily enabling tool changes to be made in vertical milling machines. Vertical milling machines usually found in machine shops are large structures that precisely support a cutting tool adjacent a movable table on which material to be milled is positioned. The tool is coupled, through a split collet, to a hollow spindle that is rotatably mounted in the machine housing. (The spindle is also adapted for limited vertical movement.) The spindle is coupled to a drive motor through a pulley arrangement to permit a variety of different spindle speeds. The split tool-holding collet has a tapered body that cooperates with a tapered opening in the spindle end to secure it in the spindle. The collet also has a threaded end that engages a so-called drawbar that passes through the hollow spindle. The drawbar generally has a hexagonally-shaped drive head that is accessible at the top of the milling machine. The drive head is turned by means of a wrench or the like to draw the collet up into the tapered opening at the lower end of the spindle. The collet, in turn, has an opening in which a cutting tool is positioned. As the collet is drawn into the spindle by the drawbar, the tapered surfaces on the collet and spindle cooperate to firmly secure the tool and collet in the spindle. This arrangement is well-known in the art.
The drive head of the drawbar is difficult to reach. Consequently, tool changing is, at best, inconvenient. Installation of a new tool is generally easier than removal, because the collet is not stuck in the spindle, a situation that often occurs in practice. The new tool is merely inserted in the collet, which is held in the spindle opening while the drive head of the drawbar is turned to draw the collet into tight frictional engagement with the spindle. A minor difficulty is that the spindle brake, which is mechanically operable to rapidly stop a rotating spindle, may require engagement to keep the spindle stationary as the drawbar is turned.
Removal of a tool is, however, a much more difficult matter. Since the collet and tool are drawn up into the spindle very tightly, it is often difficult to break the collet free from the spindle to remove a tool. Turning of the drive head merely results in the drawbar being unthreaded from the collet and protruding above the top of the milling machine. The practice is to physically hammer on the drive head to force the collet out of engagement with the spindle. While this procedure is generally satisfactory in breaking the collet loose from the spindle, it is inconvenient and time-consuming and often performed "blind," that is, the machinist cannot see the drawbar drive head since the milling machine stands quite tall.
Tool changing thus involves having a suitable wrench available to enable the drive head of the drawbar to be turned for tightening or loosening the collet in the spindle. A hammer for breaking the drawbar loose as above-mentioned is also desirable, although the wrench is often used as a hammer. In many instances, a machinist will perform operations with the wrong sized tool because of the difficulty and time to change tools. Thus, a machinist may take multiple cuts with a small tool rather than taking a large cut with a correct size tool and then changing to the smaller tool for further machining operations. All of these factors add to the inconvenience to the machinist and, of course, to the cost of the job.
An automatic mechanism for tightening and loosening tool collets is available and consists of an air-operated, rotary impact wrench that is mounted in a suitable housing on top of the vertical milling machine, with the socket of the impact wrench engageable with thread drive head of the drawbar. The mechanism generally works but is very expensive. It also has a significant disadvantage in that the very expensive top casting of the vertical milling machine can easily be damaged if the tool and collet are vertically obstructed while loosening the collet. This is because the drive head of the drawbar is levered (through the housing) against the top casting of the milling machine and, as the drawbar is unthreaded from the collet, the collet is driven downward. Any obstruction, such as work positioned under the tool, can easily result in a very large force being applied to the top casting of the milling machine. This situation has caused the destruction of a number of such castings, with the attendant loss of the machine availability and cost of repair.
With the apparatus of the invention, a small rotary impact tool is manually moved into engagement with the drive head of the drawbar and actuated under control of the operator. Not only is the danger of cracking the top casting eliminated, but much better control of the tool changing process is obtained. If a sticking collet is encountered, the impact tool may be easily used as a hammer by rapidly bringing it down into engagement with the drive head.
Another problem with the automatic mechanism is the potential danger it poses for the operator. The automatic apparatus is push-button operated and it is possible to inadvertently operate the mechanism while one's hand is positioned under the cutting tool, with potentially disastrous consequences. This potential for very serious injury, coupled with its high cost and susceptibility to causing significant damage to the milling machine, has precluded the automatic tool changing device from achieving any substantial success in the marketplace.